Rett Syndrome (RTT) is a pervasive neurodevelopmental disorder that results in mental retardation, motor dysfunction, seizures, and features of autism. RTT is caused by mutation in methyI-CpG binding protein 2 (MeCP2), a member of a family of transcriptional repressors that bind methyI-CpG DNA base pairs. Methylation is developmentally regulated, and is one mechanism whereby certain genes can be silenced to permit transition through specific developmental programs. Understanding the neuronal dysfunction that results from a mutation in a transcriptional regulator such as MeCP2 may be quite daunting, due to the complexity of the CNS and the large number of genes potentially affected. Olfactory receptor neurons (ORNs), that exist within the nasal cavity and are accessible for biopsy, display the most robust and functional post-natal neurogenesis of any neuronal population, suggesting that this system maybe useful in studying RTT. We have used olfactory nasal biopsies from RTT patients unde18 yrs old to describe that defects in neurogenesis occur in ORNs of RTT patients, resulting in a compensatory increase in neuronal precursors. We have explored the role of MeCP2 in neuronal homeostasis using the MECP2 null mouse to demonstrate that MeCP2 deficiency actually causes depletion of neuronal precursors over time. We hypothesize that MeCP2 dysfunction in RTT is different than MeCP2 deficiency: MeCP2 dysfunction permits the transition to maturity but impairs neuronal survival, while MeCP2 deficiency impairs neuronal terminal differentiation prior to synaptogenesis. We will utilize nasal biopsies containing ORNs, primary cultures of these neurons, and post-mortem olfactory bulbs from older RTT patients to elucidate the consequences of MeCP2 dysfunction versus deficiency. In Aim 1, we will perform double-labeling immunohistochemistry for stage-specific ORN markers and TUNEL to determine whether there is a shift in the ORN population in older patients and compare these results to the type of mutation present. In Aim 2 we will perform double-labeling confocal microscopy to assess synaptogenesis using axonal- and dendritic-specific markers. In Aim 3, we will establish primary cultures of ORNs from RTT patients and controls to determine whether similar maturational arrests occur, or whether neurons can mature in culture.